Liquid crystal displays and organic EL displays are widely used as various kinds of image display devices of mobile phones, car navigation devices, personal computer monitors, televisions and so on. In the liquid crystal display device, a polarizing plate is disposed on a viewing side surface of an image display cell in accordance with the display principle of the liquid crystal display device. In the organic EL display device, a circularly polarizing plate (a laminate of a polarizing plate and a quarter wave plate) may be disposed on a viewing side surface of an image display cell for inhibiting external light reflection at a metal electrode (cathode) from being viewed like a mirror surface.
In a general image display device, a polarizing plate is disposed on the outermost surface of an image display panel (a liquid crystal panel or an organic EL panel). On the other hand, a front transparent plate (also referred to as a “window layer” etc.) such as a transparent resin plate or a glass plate may be provided on a viewing side of an image display panel for the purpose of, for example, preventing damage to the image display panel due to impact from the outer surface. In a display device including a touch panel, generally the touch panel is disposed on a viewing side of an image display panel (hereinafter, the front transparent plate and the touch panel may be referred to together as a “front transparent member”).
For arranging a front transparent member such as a front transparent plate or a touch panel on a viewing side of an image display panel, an “interlayer filling structure” is employed in which the front transparent member and the image display panel are bonded with a pressure sensitive adhesive layer therebetween. In the interlayer filling structure, a gap between the panel and the front transparent member is filled with a pressure sensitive adhesive to decrease a refractive index difference at the interface, and therefore deterioration of visibility due to reflection and scattering is suppressed. There has been proposed a film with a pressure sensitive adhesive on both sides, which includes an optical film such as a polarizing plate, one surface of which is provided with a pressure sensitive adhesive layer for bonding the film to an image display panel and the other surface of which is provided with an interlayer filling pressure sensitive adhesive for bonding the film to a front transparent member (e.g., JP-A-2014-115468).
A colored layer (decorative printed layer) intended for decoration and light shielding is formed on the peripheral edge of a surface of the front transparent member on the panel side. When the decorative printed layer is formed on the peripheral edge of the transparent plate, a printing level difference of about 10 μm to several tens μm is generated. When a sheet pressure sensitive adhesive is used as an interlayer filler, bubbles are easily generated on the periphery of the printing level difference portion. Display unevenness may occur at the peripheral edge of a screen because local stress is added to the image display panel immediately below the printing level difference portion through the pressure sensitive adhesive, so that the end of the screen is dynamically distorted.
For solving problems caused by a printing level difference of a front transparent member as described above, a soft and thick pressure sensitive adhesive sheet is used for bonding the front transparent plate, so that level difference absorbency is imparted. For example, JP-A-2014-115468, JP-A-2011-74308 and JP-A-2010-189545 describe that the storage elastic modulus and residual stress of an interlayer filling pressure sensitive adhesive layer to be used for bonding an optical film and a front transparent member are each made to fall within a specific range.